Dual flash drive boot with boot loader (grub)

I am using a Thinclient, which has 2 flash drives. There is no hardware RAID1 implemented. The requirement is that, whenever 1 flash drive goes down, we should be able to boot and continue from the other flash drive. Details : I have the following scenario : My machine has 2 flashdrive cards Scenario 1 : Machine has the first flashdrive plugged in, say D1 (/dev/sda) If you boot the machine, the following happens : - Machine ->bios ->grub ->ubuntu (all is well). The flash drive was installed with Ubuntu through a usb device (lsblk shows /dev/sda and /boot/efi is mounted on /dev/sda1) UNPLUG D1 PLUG IN DRIVE D2 (/dev/sdb) If you boot the machine, the following happens : - Machine ->bios ->grub ->ubuntu (all is well). The flash drive was installed with Ubuntu through a usb device (not “dd” etc) (lsblk shows /dev/sdb and /boot/efi is mounted on /dev/sdb1) PLUG IN D1 AGAIN Booting sequence defined in bios, in the order of D1 and then D2. - Machine gets stuck in bios, does not goto grub and hangs in the bios *It should have booted from D1 (lsblk shows /dev/sda and /dev/sdb. The /boot/efi is mounted on /dev/sdb1) Additional Info : The flash drives are identical. We are using a Thinclient which comes inbuilt with both the flash drives. The bios version is 3.0E Each drive has a diff UUID